1. Field of the Invention
The present invention relates to a fuel injection valve mainly used for a fuel supply system of an internal-combustion engine, and specifically to an injection hole plate provided at the downstream side of a valve seat.
2. Description of the Related Art
Recently, with tighter exhaust emission regulation for vehicles and the like, improvements in the degree of freedom of the injection direction and atomization of the fuel spray injected from the fuel injection valve have been required, and particularly, various studies have already been made on the atomization of the fuel spray.
In a fuel injection valve in related art, a fuel injection valve in which a thin injection hole plate with plural injection holes formed therein is provided at the downstream side of a valve member and a valve seat for injecting fuel from the respective injection holes is known. In the fuel injection valve, generally, the injection hole has an equal diameter from its entrance to exit, and, when fuel flows into the injection hole having the equal diameter, the fuel does not spread along the injection hole inner surface but is injected as a liquid column. The fuel as the liquid column is hard to be atomized and may deteriorate combustion in the internal-combustion engine.
In this regard, for example, as shown in Patent Document 1, in order to obtain a liquid membrane sufficiently spread within the injection hole, it is conceivable that the injection hole is tapered. By forming a conical shape spreading at the exit side of the injection hole, it may be expected that the spray spreads along the inner wall surface of the injection hole, and the fuel turns into a thin membrane and is injected.
Further, for example, as shown in Patent Document 2, the fuel entrance is formed in an elliptical shape or plural circles and the fuel is injected from a belt-like exit that smoothly communicates with the entrance, and thus, the spray in the uniform liquid membrane may be injected from the exit and atomization of the fuel may be promoted.
Furthermore, for example, as shown in Patent Document 3, the fuel spray may be atomized by forming an oval injection hole with a major axis and a minor axis to have an injection hole shape with a straight elongated shape and arc shapes on both ends, and making the minor axis shorter. Further, as a modified example of the oval injection hole, three circular injection holes may be aligned on a straight line to form one injection hole.    [Patent Document 1] JP-A-2001-317431 (paragraph 0019, FIG. 4)    [Patent Document 2] JP-A-2006-2720 (paragraph 0061, FIGS. 9 and 11)    [Patent Document 3] JP-A-8-200187 (paragraph 0021, FIGS. 1 and 5)
However, in the case where the tapered injection holes as shown in Patent Document 1 are formed in the injection hole plate, the areas of injection hole entrances may easily vary in respect of working, and there is a problem of variations in flow rate and spray, and further, very complex steps are necessary in respect of manufacture and dimensional management, and there are problems that deterioration of productivity of the fuel injection valve and increase in cost may be caused.
In addition, the injection holes shown in Patent Document 2 and Patent Document 3 have a structure of injecting spray in a shape along a channel shape while filling the injection hole with fuel. Accordingly, there are problems that air bubbles produced in the fuel due to decompression boiling at the downstream side of the injection hole at a high temperature under negative pressure are clogged at the injection hole exit, and flow characteristics (static flow, dynamic flow) with changes in temperature, atmosphere pressure, etc. largely change.